Advantages of Water-Cooled Furnaces

The furnace is completely water cooled except for the opening for manholes and burn- ers as shown. The header system of feeding the front wall from the lower drum with water and taking the steam water mixture to the steam drum and the O-ring for accom- modating the burner is a recent development. This design developed by Nebraska Boiler Company has been in use in several hundred boilers worldwide. The O-ring concept is also an elegant way of providing an opening for the burner compared to the complex bent tube design. In order to ensure that steam bubbles are pushed upward, twisted steel strips are placed in the O-ring. This concept is similar to the rifled tubes used in high-pressure boilers to improve the wetting of the tube periphery by water and hence increasing the limit of critical heat flux (CHF). To improve circulation in the front wall, an orifice of cal- culated dimension is placed in the D header. Circulation calculation for boilers with such

a design is shown elsewhere. The advantages of the completely water-cooled furnace are as follows:

1. The front, rear, and side walls are completely water cooled and are of membrane

wall construction, resulting in a leak-proof enclosure for the flame. The entire furnace expands and contracts uniformly, thus avoiding casing expansion concerns. When refractory is used on the front, side, or rear wall, the sealing between the hot- ter membrane wall and the refractory casing is difficult, and hot gases can some- times leak from the furnace to the outside as the furnace is pressurized. This can cause corrosion of the casing, particularly if oil fuels containing sulfur are fired.

2. Problems associated with refractory maintenance are eliminated. Also there is no need for annual shutdown for maintenance of the boiler plant to inspect or repair the refractory, thus lowering the cost of owning the boiler.

3. Fast start-up and shutdown rates are difficult with refractory-lined boiler because of the possibility of causing cracks in the refractory. However, with the completely water-cooled furnace, start-up rates are limited only by the thermal stresses in the drums and are much quicker; the evaporator and furnace tubes may be welded to the drums instead of being rolled if start-ups are frequent. With boilers maintained in hot standby conditions using steam-heated coils located in the mud drum, even 5–10 min start-ups are feasible. With a separate burner whose capacity is about 5%–7% of the main burner, the boiler can be maintained hot or at pressure. This concept has been used in a few quick start boilers in cogeneration plants where the boiler is required to come on line to full load within 2–3 min of firing. Hence, maintenance costs and fuel costs during each start-up are reduced, benefit- ting the end user in the long run.

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4. Heat release rates on area basis will be lower by about 7%–15% for the water- cooled furnace for a given volume compared to the refractory-lined boiler due to the absence of refractory on front or rear walls and floor. Some oil- and gas-fired boilers designed several decades ago still have refractory on the floor, making the furnace less effective. Modifying the boiler to replace refractory with membrane wall will make the furnace more effective and lower the heat flux marginally.

A lower area heat release rate (AHRR) also helps lower NO x as can be seen from the correlations developed by a few burner suppliers.

5. Reradiation from the refractory on the front wall, side walls, and the floor increases the flame temperatures locally, which results in higher NO x formation. Of the total NO x generated by the burner, a significant amount is formed in the burner flame base; hence, providing a cooler environment for the flame near the flame helps lower NO x .

6. Circulation of steam water mixture in all the sections of the boiler, particularly the floor tubes, was a concern decades ago, which prompted many boiler suppliers to place refractory over the floor tubes. With better furnace designs, operational experience, and advanced burner designs, refractory is not used nowadays.

Heat fluxes in packaged boilers are generally low compared to those of utility boilers. To further protect the floor and roof tubes, tubes are inclined about 3°–5° to the horizontal. Also considering the low steam pressure (less than 110 barg), circulation has never been an issue as evidenced by the operation of hundreds of boilers firing oil and gas and generating up to 200 t/h of steam. The tube- side velocities are also reasonable to ensure that stagnation does not occur in the tubes, and hence, refractory on the floor is not required. Besides, burner suppliers have improved their design significantly in the past decade, and concerns about oil spilling on the floor causing local burning have been addressed. Furnace dimensions are also chosen such that the flame does not impinge on the furnace enclosure.

7. Package boilers today do not use air heaters for heat recovery. Economizers alone are used. Use of ambient air versus hot air lowers the combustion temperature in the furnace and thus lowers NO x . The air- and gas-side pressure drops are also lower without the air heater, thus reducing the fan size and operating cost.

Advantages of Water-Cooled Furnaces